The present invention relates to the field of mass storage devices. More particularly, this invention relates to an improved head suspension assembly of a disc drive.
One key component of any computer system is a device to store data. Computer systems have many different places where data can be stored. One common place for storing massive amounts of data in a computer system is on a disc drive. The most basic parts of a disc drive are an information storage disc that is rotated, an actuator that moves a transducer to various locations over the disc, and electrical circuitry that is used to write and read data to and from the disc. The disc drive also includes circuitry for encoding data so that it can be successfully retrieved and written to the disc surface. A microprocessor controls most of the operations of the disc drive as well as passing the data back to the requesting computer and taking data from a requesting computer for storing to the disc.
The transducer is typically placed on a small ceramic block, also referred to as a slider that is aerodynamically designed so that it flies over the disc. The slider is passed over the disc in a transducing relationship with the disc. Most sliders have an air-bearing surface (ABS) which includes rails and a cavity between the rails. When the disc rotates (generally, at rotational speeds of 10,000 RPM or higher), air is dragged between the rails and the disc surface causing pressure, which forces the head away from the disc. At the same time, the air rushing past the cavity or depression in the air-bearing surface produces a negative pressure area. The negative pressure or suction counteracts the pressure produced at the rails. The slider is also attached to a load spring, which produces a force on the slider directed toward the disc surface. The various forces on the slider equilibrate, so that the slider flies over the surface of the disc at a particular desired fly height. The fly height is the distance between the disc surface and the transducing head, which is typically the thickness of the air lubrication film. This film eliminates the friction and resulting wear that would occur if the transducing head and disc were in mechanical contact during disc rotation. In some disc drives, the slider passes through a layer of lubricant rather than flying over the surface of the disc.
Information representative of data is stored on the surface of the storage disc. Disc drive systems read and write information stored on tracks on storage discs. Transducers, in the form of read/write heads attached to the sliders, located on both sides of the storage disc, read and write information on the storage discs when the transducers are accurately positioned over one of the designated tracks on the surface of the storage disc. The transducer is also required to be moved to a target track. As the storage disc spins and the read/write head is accurately positioned above a target track, the read/write head can store data onto a track by writing information representative of data onto the storage disc. Similarly, reading data on a storage disc is accomplished by positioning the read/write head above a target track and reading the stored material on the storage disc. To write on or read from different tracks, the read/write head is moved radially across the tracks to a selected target track. The data is divided or grouped together on the tracks. In some disc drives, the tracks are a multiplicity of concentric circular tracks. In other disc drives, a continuous spiral is one track on one side of the disc drive. Each track on a disc surface in a disc drive is further divided into a number of short arcs called sectors. Servo feedback information is used to accurately locate the transducer head on to the tracks/sectors. The actuator assembly is moved to the required position and held very accurately during a read or write operation using the servo information.
The actuator assembly is composed of many parts that contribute to the performance required to accurately hold the read/write head in the proper position. There are two general types of actuator assemblies, a linear actuator and a rotary actuator. The rotary actuator includes a pivot assembly, an arm, a voice coil yoke assembly, and a head gimbal suspension assembly. The rotary actuator assembly pivots or rotates to reposition the transducer head over particular tracks on a disk. A suspension or load beam is part of the head gimbal suspension assembly. The rotary actuator assembly also includes a main body, which includes a shaft and bearing about which the rotary actuator assembly pivots. Attached to the main body are one or more arms. One or typically two head gimbal suspension assemblies are attached to the arm. Generally the length of the arm is approximately equal to the length of the suspension. Total length of the arm and the suspension is one of many other factors that can affect mechanical resonance frequencies of the actuator arm assembly.
One end of the suspension is attached to the actuator arm assembly. The transducer head, also known as a read/write head, is found attached to the other end of the suspension. One end of the actuator arm assembly is coupled to a pivot assembly. The pivot assembly, in turn, is connected to a voice coil motor attached to a voice coil yoke on the main body of the actuator assembly. The other end of the actuator arm assembly is attached to the head gimbal suspension assembly. The head gimbal suspension assembly includes a gimbal to allow the read/write head to pitch and roll and follow the topography of the imperfect memory disc surface. The head gimbal assembly also restricts motion with respect to the radial and circumferential directions of the memory disc. The suspension assembly is coupled to the actuator arm assembly as part of the main body of the actuator assembly, which holds the pivot support and is coupled to the voice coil motor.
Actuator arms are cantilevered assemblies, which act as spring-mass-damper systems, and have resonant frequencies that can degrade the performance of the servo system. Every closed loop servo motor system has a predetermined bandwidth in which mechanical resonances occurring within the bandwidth degrade the performance of the servo motor system. The actuator arm is one key source of unwanted mechanical resonances. Accordingly, the bandwidths of most servo motor systems are designed so that resonance of the actuator arm and suspension occur outside the bandwidth. Current actuator arms are made of steel, aluminum or magnesium. Suspensions are typically made of stainless steel.
Increasingly, higher number of bits are being packed into every square inch of the disc surface leading to a higher number of tracks per inch (TPI) or reduced track width. Thus, the head (and thus the suspension) needs to track follow more effectively or have further reduced off-track motion. In order to accomplish this, a higher servo bandwidth is required. To develop servo systems with a higher bandwidth, the suspension resonance frequencies need to be increased (a stiffer suspension is required).
The resonant characteristics of the actuator arm have bending and torsion modes, with frequencies that are within the same frequency range as the suspension and the magnetic storage disc. Great care must be used when designing an actuator system to prevent alignment of resonance modes that would create very high gains and an unstable servo performance. Alignment of resonance modes means, one component resonates at a frequency, which is very near, or the same as the resonant frequency of another component.
Actuator arms and suspensions can be made thicker to increase the bending and torsion mode frequencies, but the greater mass significantly degrades the performance of the actuator assembly by increasing the moment of inertia of the arm. Inertial increase will increase the access time for moving the transducer between data tracks. Yet another problem of increasing the arm and suspension thickness is, it can increase the current requirements necessary to move the voice coil motor. Increased current results in increased heat within the disk enclosure and increased power requirements.
Use of thicker sheet material and a reduced effective bend length for the actuator arm and the suspension can also result in a very high vertical stiffness (spring rate). However, this can result in requiring additional rework in the head stack assembly process to achieve the desired or target gram load. Typically, the suspension assembly has a spring section, which includes a preformed bend or radius. This radius provides the spring or load force and thus a desired load to the head slider for a predetermined offset height, the offset height being a measurement of the distance between the mounting height of the head suspension to the actuator and the head slider at xe2x80x9cflyxe2x80x9d height. Spring force (is the force the spring region exerts on the head slider toward the disc) is directly proportional to the distance the head slider has been deflected away from the disc. The greater the deflection, the greater the opposing force, and lesser the deflection the lower the opposing force.
As this discussion makes clear, the fly height of the head slider above the disc is a balance of the lifting force and the opposing load force. Thus, the load force is one factor that directly determines the height at which the head moves over the disc. This height is critical to high speed, accurate storage and retrieval of data. Disc drive manufacturing processes can make fly-height control difficult to realize due to handling of suspension after production and manufacturing tolerances within the disc drive manufacture and/or assembly. A tight control of the gram load is required to achieve the appropriate fly height of the head in the drive and reduce the fly-height variation. Prior art reveals a method of reducing spring rate of the suspension by etching out approximately 50-60% of the material in the preformed bend area (load carrying section) of the suspension. Although, this method leads to a reduction in the spring rate, the variation in the remaining material thickness dictated by the etching process can result in a significant variation in the spring rate, the free state angle, and the suspension resonances. In addition, the etching process can generate sharp corners in the preload bend radius which, can in-turn result in regions of stress concentrations. This can in turn, lead to a lower force to yield, making the suspension more prone to gram load loss during assembly process as well as in the drive.
What is needed is an improved head suspension assembly, for a disc drive, that increases stiffness-to-mass ratio without increasing the spring rate of the head suspension assembly so that the suspension resonance frequencies of the head suspension assembly fall outside the bandwidth of the servo drive to reduce off-track motion of the head slider, during track follow-and-seek operations, to meet the industry""s ever increasing need to store higher number of bits for every square inch of a disc surface of the disc drive.
A disc drive includes a base and a disc rotatably attached to the base. The disc drive also includes an actuator arm assembly attached to the base such that the actuator arm assembly is in an actuating relationship with respect to the base and the rotating disc. A servo drive controls the movement of the actuator arm assembly during track follow-and-seek operations of the disc drive. An improved head suspension assembly having a higher stiffness-to-mass ratio to increase suspension resonance frequencies to fall outside a bandwidth of the servo drive without increasing spring rate of the head suspension assembly is attached to the actuator arm assembly to reduce off-track motion of a transducer head/slider during the track follow-and-seek operations of the disc drive. The improved head suspension assembly includes a base plate, a two-piece suspension member, and a gimbal. The two-piece suspension member includes a first and second pieces. The base plate is attached to the actuator arm assembly. The first piece of the two-piece suspension member is attached to the base plate such that the two-piece suspension member, the base plate, and the actuator arm assembly are all in an actuating relationship with respect to the rotating disc. Next, the gimbal is attached to the first and second pieces of the two-piece suspension member such that a predetermined bendable area having stiffer preload bend radius is formed between the first and second pieces. The stiffer preload bend radius in the gimbal provides a higher stiffness-to-mass ratio to increase suspension resonance frequencies such that the suspension resonance frequencies fall outside a bandwidth of the servo drive without increasing the spring rate of the suspension assembly.
Also, discussed is a method of increasing suspension resonance frequencies without increasing spring rate of the head suspension assembly of a disc drive. The method begins with the step of attaching a base plate to an actuator arm assembly of the disc drive. Next, the method includes attaching the base plate to a two-piece suspension member having first and second pieces such that the base plate including the two-piece suspension member are in an actuating relationship with respect to a rotating disc of the disc drive. Then, the method includes attaching a gimbal to the two-piece suspension member such that the gimbal has a predetermined bendable area having stiffer preload bend radius between the first and second pieces. The stiffer preload bend radius provides a higher stiffness-to-mass ratio to the head suspension assembly to increase suspension resonance frequencies such that the suspension resonance frequencies fall outside a bandwidth of a servo drive to reduce off-track motion during track follow-and-seek operations of the disc drive.
Advantageously, the method and the apparatus described above increase the stiffness-to-mass ratio to increase the suspension resonance frequencies of the head suspension assembly such that the suspension resonance frequencies fall outside the bandwidth of the servo drive to reduce off-track motion of transducer heads/sliders during track follow-and-seek operations. This is accomplished without increasing the spring rate of the head suspension assembly. As a result, the disc drive can achieve an improved resonance performance, a higher servo bandwidth, and thus a better track following capability. This can result in having higher track densities, and increased storage capability in the disc drive.